The present invention relates to wireless communications, and more particularly to a wireless communication system configured to communicate using a single-carrier to multi-carrier mixed waveform configuration.
The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 standard is a family of standards for wireless local area networks (WLAN) in the unlicensed 2.4 and 5 Gigahertz (GHz) bands. The current 802.11 b standard defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Megabits per second (Mbps). The 802.11b standard uses direct sequence spread spectrum (DSSS) with a chip rate of 11 Megahertz (MHz), which is a serial modulation technique. The 802.11a standard defines different and higher data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band. It is noted that systems implemented according to the 802.11 a and 802.11b standards are incompatible and will not work together.
A new standard is being proposed, referred to as 802.11 g (the xe2x80x9c802.11 g proposalxe2x80x9d), which is a high data rate extension of the 802.11b standard at 2.4 GHz. It is noted that, at the present time, the 802.11 g proposal is only a proposal and is not yet a completely defined standard. Several significant technical challenges are presented for the new 802.11 g proposal. It is desired that the 802.11 g devices be able to communicate at data rates higher than the standard 802.11b rates in the 2.4 GHz band. In some configurations, it is desired that the 802.11b and 802.11 g devices be able to coexist in the same WLAN environment or area without significant interference or interruption from each other, regardless of whether the 802.11b and 802.1 g devices are able to communicate with each other. It may further be desired that the 802.11 g and 802.11b devices be able to communicate with each other, such as at any of the standard 802.11b rates.
A dual packet configuration for wireless communications has been previously disclosed in U.S. patent application entitled, xe2x80x9cA Dual Packet Configuration for Wireless Communicationsxe2x80x9d, Ser. No. 09/586,571 filed on Jun. 2, 2000, which is hereby incorporated by reference in its entirety. This previous system allowed a single-carrier portion and an orthogonal frequency division multiplexing (OFDM) portion to be loosely coupled. Loosely coupled meant that strict control of the transition was not made to make implementations simple by allowing both an existing single-carrier modem and an OFDM modem together with a simple switch between them with a minor conveyance of information between them (e.g., data rate and packet length). In particular, it was not necessary to maintain strict phase, frequency, timing, spectrum (frequency response) and power continuity at the point of transition (although the power step would be reasonably bounded). Consequently, the OFDM system needed to perform an acquisition of its own, separate from the single-carrier acquisition, including re-acquisition of phase, frequency, timing, spectrum (including multi-path) and power (Automatic Gain Control [AGC]). A short OFDM preamble following the single carrier was used in one embodiment to provide reacquisition.
An impairment to wireless communications, including WLANs, is multi-path distortion where multiple echoes (reflections) of a signal arrive at the receiver. Both the single-carrier systems and OFDM systems must include equalizers that are designed to combat this distortion. The single-carrier system designs the equalizer on its preamble and header. In the dual packet configuration, this equalizer information was not reused by the OFDM receiver. Thus, the OFDM portion employed a preamble or header so that the OFDM receiver could reacquire the signal. In particular, the OFDM receiver had to reacquire the power (AGC), carrier frequency, carrier phase, equalizer and timing parameters of the signal.
Interference is a serious problem with WLANs. Many different signal types are starting to proliferate. Systems implemented according to the Bluetooth standard present a major source of interference for 802.11-based systems. The Bluetooth standard defines a low-cost, short-range, frequency-hopping WLAN. Preambles are important for good receiver acquisition. Hence, losing all information when transitioning from single-carrier to multi-carrier is not desirable in the presence of interference.
There are several potential problems with the signal transition, particularly with legacy equipment. The transmitter may experience analog transients (e.g., power, phase, filter delta), power amplifier back-off (e.g. power delta) and power amplifier power feedback change. The receiver may experience AGC perturbation due to power change, AGC perturbation due to spectral change, AGC perturbation due to multi-path effects, loss of channel impulse response (CIR) (multi-path) estimate, loss of carrier phase, loss of carrier frequency, and loss of timing alignment.
A wireless communication system configured to communicate using a mixed waveform configuration is disclosed and includes a transmitter configured to transmit according to a mixed waveform configuration and a receiver configured to acquire and receive packets with a mixed waveform configuration. The mixed waveform includes a first portion modulated according to a single-carrier scheme with a preamble and header and a second portion modulated according to a multi-carrier scheme. The waveform is specified so that a channel impulse response (CIR) estimate obtainable from the first portion is reusable for acquisition of the second portion.
In one configuration, the transmitter maintains power, carrier phase, carrier frequency, timing, and multi-path spectrum between the first and second portions of the waveform. The transmitter may include first and second kernels and a switch. The first kernel modulates the first portion according to the single-carrier modulation scheme and the second kernel generates the second portion according to the multi-carrier modulation scheme. The switch selects the first kernel for the first portion and the second kernel for the second portion to develop a transmit waveform. In one embodiment, the first kernel operates at a first sample rate and the second kernel operates at a second sample rate. The first kernel may employ a single-carrier spectrum that resembles a multi-carrier spectrum of the multi-carrier modulation scheme.
The first kernel may employ a time shaping pulse that is specified in continuous time. The time shaping pulse may be derived by employing an infinite impulse response of a brick wall approximation that is truncated using a continuous-time window that is sufficiently long to achieve desired spectral characteristics and sufficiently short to minimize complexity. The first kernel may sample the time shaping pulse according to a Nyquist criterion. The average output signal power of the first kernel and the average output signal power of the second kernel may be maintained substantially equal. The first kernel may employ a first sample rate clock while the second kernel employs a second sample rate clock. In this latter case, the first and second sample rate clocks are aligned at predetermined timing intervals. Also, a first full sample of the multi-carrier modulation scheme begins one timing interval after the beginning of a last sample of the single-carrier modulation scheme.
The single-carrier signal from the first kernel may be terminated according to a windowing function specified for OFDM signal shaping defined in the 802.11a standard. The carrier frequency may be coherent between the first and second kernels. The carrier phase may be coherent between the first and second kernels. In one embodiment to achieve coherent phase, carrier phase of the second kernel multi-carrier signal is determined by carrier phase of a last portion of the second kernel single-carrier signal. The carrier phase of the second kernel multi-carrier signal may further be rotated by a corresponding one of a plurality of rotation multiples, each rotation multiple corresponding to one of a plurality of predetermined phases of the last portion of the second kernel single-carrier signal. In a particular embodiment, the first kernel single-carrier modulation scheme is according to 802.11b Barkers in which each Barker word is one of first, second, third and fourth possible phases and the second kernel multi-carrier modulation scheme is according to OFDM as defined in Annex G of the 802.11a standard. In this case, the OFDM symbols are rotated by the second kernel by zero if the last Barker word has the first phase, by 90 degrees if the last Barker word has the second phase, by 180 degrees if the last Barker word has the third phase, and by xe2x88x9290 degrees if the last Barker word has the fourth phase.
The requisite fidelity of the entire mixed waveform configuration may be specified by a requisite fidelity specified for the multi-carrier scheme. In one embodiment, the requisite fidelity is a function of data rate of the second portion and is determined by mean-squared-error normalized by signal power as specified for OFDM in the 802.11a standard.
The symbol rate clock and carrier frequency of the waveform may be derived from the same reference clock. The part per million (PPM) error of a clock fundamental for symbol rate and PPM error of a clock fundamental for carrier frequency may be substantially equal.
The receiver may include a single-carrier receiver, a multi-carrier receiver, and a switch that provides a first portion of a signal being received to the single-carrier receiver and that provides a second portion of the signal being received to the multi-carrier receiver. The single-carrier receiver acquires a first portion of an incoming signal including the preamble and header and determines a CIR estimate, and the multi-carrier receiver uses the CIR estimate for a second portion of the incoming signal. In a specific configuration, the single-carrier receiver programs taps of the first equalizer based on the CIR estimate, the multi-carrier receiver includes a second equalizer, and the multi-carrier receiver modifies taps of the second equalizer based on the CIR estimate determined by the first equalizer.